A black hole, like a spinning wheel, twists a cloud of light particles, axions, around itself. These particles are the leading candidates for dark matter. The cloud follows the laws of quantum mechanics, resembling a giant atom: there are levels, and there are transitions between them. Only instead of electric forces, gravity is at play.
When such a system enters a pair with another black hole or a star, tidal forces start to rock the 'atom'. Transitions between levels give rise to gravitational waves — the trembling of spacetime. Previously, the focus was on the transition probability. The authors show that what matters more is the coherence with which the cloud radiates. If the approach is too slow, the signal fades. If it is too fast, it also fades. But at a moderate pace, a clear burst emerges that can be captured.
In more massive systems, the transitions become narrowband, like a signal of precise frequency. This leaves a distinct imprint on the gravitational waveform, distinguishing it from an ordinary merger of two bodies. Such a 'ring' can reveal the properties of axions and shed light on the nature of dark matter.
An axion cloud around a black hole can be billions of times larger than the event horizon itself, like a pea wrapped in a fog the size of Earth.
In Vernor Vinge's novel 'A Fire Upon the Deep', superluminal communication relies on quantum effects near singularities — somewhat akin to how gravitational atoms exchange signals through spacetime.